Since lithium secondary batteries have high energy density, the application ranges of lithium secondary batteries are not limited to handheld equipment, such as mobile phones and personal computers, and the like, but are expanded to hybrid automobiles, electric automobiles, electric power storage systems, and the like. As one of such lithium secondary batteries, attention has recently been paid to a lithium-sulfur secondary battery arranged to perform electric charge and discharge through reactions of sulfur and lithium by using the sulfur as a positive electrode active material and the lithium as a negative electrode active material.
The lithium-sulfur secondary battery has an advantage of achieving an improvement of the specific capacity of the lithium-sulfur secondary battery, because at most two lithium ions react with one sulfur atom and sulfur is lighter than transition metals. On the other hand, sulfur is an insulator having extremely high resistance (5×1030 Ω·cm). For this reason, it is common practice to mix sulfur with a conductive additive such as acetylene black when the sulfur is used as a positive electrode active material. In case where the acetylene black is mixed with sulfur as described above, high resistance occurs among particles of the acetylene black, whereby the provision of electrons to the sulfur tends to be insufficient. As a result, the utilization efficiency of the sulfur is lowered, which in turn causes a problem in that the specific capacity is limited.
In addition, during electric discharge of a lithium-sulfur secondary battery, cyclic sulfur S8 is broken to form straight chain sulfur S82−, and this S82− is further transformed into S62−, S42−, S32−, S22−, and S2−. These polysulfide anions S82− to S32− are dissolved in an electrolytic solution, and are diffused in the electrolytic solution. Then, upon reaching a negative electrode, the polysulfide anions react with lithium on the negative electrode to generate lithium sulfide Li2S2, Li2S. Here, the lithium sulfide is electrochemically inactive. For this reason, once deposited on the negative electrode, the lithium sulfide is not dissolved into the electrolytic solution. Consequently, the lithium-sulfur secondary battery has another problem in that the cycling characteristic is lowered.
As solutions to the foregoing problems, there have been proposed dry polymerization (gelation) in which an electrolytic solution is contained in polymers made of polyethylene oxide (PEO) or the like, or complete solidification using a sulfide solid electrolyte such as Li—P—S or Li—Si—S (see, e.g., Non-Patent Documents 1 and 2). In the above methods, even though the specific capacity and the cycling characteristic are improved, a reaction speed of lithium and sulfur is lower than in the case using the electrolytic solution. As a result, a high rate characteristic cannot be obtained.
Moreover, there has been proposed another method using as a positive electrode active material layer a composite produced by coating a current collector with a slurry in which sulfur is mixed with carbon nanotubes and acetylene black as conductive additives (see, e.g., Non-Patent Document 3). In this method, the carbon nanotubes adsorb polysulfide anions generated during the electric discharge to prevent diffusion of the polysulfide anions into the electrolytic solution, thereby improving the cycling characteristic. However, this method still has a problem in that the resistance occurring among the particles of the acetylene black imposes limitation on the rate characteristic.